Alternative fuels such as natural gas are being adopted by engine manufacturers in so called dual fuel, bi-fuel and flex-fueled engines. Dual fuel engines are fueled with more than one fuel type simultaneously, for example engines that consume natural gas as a main fuel and diesel as a pilot ignition fuel. Bi-fuel engines can consume more than one fuel type but are fueled with only one type at a time, such as engines that consume gasoline or natural gas, but not both together. Flex-fuel engines combine features of both dual fuel and bi-fuel engines and can consume more than one fuel type simultaneously or can operate on a single fuel. For example an engine that normally consumes either gasoline or natural gas but which can consume both fuels simultaneously according to engine operating conditions.
As a first step in adopting alternative fuels in directly injected gasoline engines, manufacturers integrated port injection systems that introduced a gaseous fuel such that the engine could operate as a bi-fuel or flex-fuel engine. An exemplary alternative fuel for these engines was natural gas. Since natural gas was considered an alternative fuel and not a primary fuel, these engines were optimized for operation with gasoline. When the engine was operating with natural gas it resulted in reduced power and torque compared to operation with gasoline. One reason for this reduced performance is the displacement of air by natural gas during the intake stroke. By displacing air in the cylinder there is less oxidant available for combustion with fuel, which reduces the average combustion pressure and power available to propel the vehicle.
As gasoline continues to increase in cost relative to natural gas and emission regulations become more stringent, engine manufacturers are motivated to improve bi-fuel and flex-fuel engines, in addition to mono-fuel engines, for operation on natural gas. One known technique for increasing power and performance in engines that consume natural gas is to introduce the gas at high pressure. This requires pressurizing natural gas to a high injection pressure, which involves extra components such as pumps, heat exchangers and high pressure piping that increase the cost of the fueling system.
Natural gas is a compressible fluid that takes considerably more energy and time to pressurize than gasoline, which is an incompressible fluid. In some applications the increased cost and extra time is offset by other economic and logistical factors, but in other applications it is not acceptable. Other techniques to improve natural gas operation in engines employed outward opening direct injectors. These injectors had inadequate flow rates that influenced the technique of introducing natural gas to the cylinders. The mixing of natural gas with the trapped charge was poor and resulted in lost combustion efficiency and reduced performance at higher engine speeds where the time windows available for injection and mixing were shorter.
United States Patent Application, Publication Number US 2003/0140902 A1, entitled “CNG Direct-Injection Into IC Engine” discloses a method for operating a four-stroke internal combustion engine with natural gas. The '902 application discloses whereby at high to full torque operating conditions, the natural gas is injected directly into the cylinder of the engine toward the end of the induction stroke or at the start of the compression stroke. This method claims to be able to inject the gaseous fuel without needing to generate a gas pressure higher than the storage pressure. However, a disadvantage of this system is that at such high load conditions, the engine consumes more fuel, and with a low gaseous fuel injection pressure it can be difficult to inject enough fuel into the cylinder early enough to allow sufficient time for mixing with the charge in the cylinder. The '902 application also claims that one of the advantages of its approach is that “switching to induction of fuel with the air is also avoided.”
The state of the art is lacking in techniques for introducing gaseous fuel to an internal combustion engine as a function of engine operating conditions for improved performance.